In order to provide sealing in the region of the aperture in the external transverse face of the casing, through which the starter head extends, it is known to employ a seal having a lip giving radial frictional contact, the lip being fixed on the casing of the starter around the aperture, with a lip of the seal bearing on a cylindrical engagement surface of the starter head.
This arrangement has the disadvantage that it leaves the lip of the seal in permanent contact with the engagement surface of the starter head. Thus, when the starter head is displaced axially from its rest position towards its working position, the friction of the lip of the seal on the cylindrical engagement surface of the starter head acts as a brake on the displacement of the head. This can cause the starter motor, and therefore the starter head, to be put into rotation before the starter pinion of the head has become properly meshed with the toothed starter crown carried by the inertia flywheel of the engine of the vehicle.
In addition, when the starter head is rotating about its axis, the lip of the seal continues to rub frictionally against the starter head; moreover, this friction is produced at high speed, which leads to rapid deterioration of the lip and thus results, before long, in poor sealing.
With a view to overcoming the first of these problems, it has been proposed to provide a sealing gasket of the flat type, which is adapted to engage, when the starter head is in its rest position, against the external transverse face of the casing of the starter. This arrangement does however having the disadvantage that it makes it necessary for the flat sealing gasket to act as an axial positioning abutment for the starter head in the rest position, so as to achieve good contact of the flat seal on the said external transverse face, regardless of the geometric variations which are inherent in the construction of the starter.
With that arrangement, when the starter head returns from its working position to its rest position after having started the engine of the vehicle, but while it is still rotating about its axis at a high speed, the flat gasket is in frictional movement against the external transverse face of the casing. This arrangement therefore does not resolve the problem of wear of the seal due to friction.